PROTOZOA. 



shells of various external form (like helmets, bird-cages, shells, 

 etc.) are found, and on the periphery of these, spicules and needles, 

 and even external concentric shells of similar shape may be formed, 

 e.g., Polycystina. (Figs. 18 and 19.) 



Up to the present time but little has been made out about the 

 reproduction of these animals. Besides fission of the central capsule 

 (Polycyttaria), the formation of spores has been observed. These are 

 formed from the contents of the central capsule, and, after the burst- 

 ing of the latter, become free-swimming mastigopods. The spore- 

 formation is of two kinds : in the one it results in the development 



of a mastigopod contain- 

 ing .a crystal the crys- 

 tallit/erous swarmer ; in 

 the other (dimorphous) 

 two kinds of swarmers are 

 formed the macrospore's 

 and microspores, being 

 distinguished from one 

 another by their size. 

 The further history of 

 the spores is not known. 

 Conjugation has not been 

 observed in the Radio- 

 laria ; but it has been 

 suggested that the macro- 

 spores and microspores 

 may turn out to be conju- 

 gating cells. Radiolaria 

 are inhabitants of the sea, and swim at the surface, but some live 

 011 the bottom, even at great depths. 



Fossil remains of Radiolaria have been made known in great 

 numbers by Ehrenberg, e.</., from the chalky marl and polishing slate 

 found at certain parts of the coast of the Mediterranean (Caltanisetta 

 in Sicily, Zante and ^Egina in Greece), and in particular from the 

 rocks of Barbados and Xikobar, where the Radiolaria have given 

 rise to widely extended rock formations. Samples of sand also 

 from very considerable depths have shown themselves rich in 

 Radiolarian shells. 



Order 1. PERIPYLAKIA. Central capsule uniformly perforated by numerous 

 fine pores, with or without silicious skeleton. 



Fain. 1. Colloidea without skeleton. Solitary are TJialassolampeH., ThoJasso- 



FIG. IS. Heliosphcera echinoides (after B. Haeckel). 



